1. Field of the Invention
The present invention relates to an optical fiber unit in which at least 1-core optical fiber is mounted in and protected by a buffer tube, and an optical cable including the optical fiber unit.
2. Description of the Related Art
As various communication service such as bi-directional communications and image communications is propagated more broadly at home and abroad, a demand for optical cables keeps increasing. Referring to FIG. 1, a general optical cable 20 has a plurality of optical fiber units 10 in which 1- to 12-core optical fiber 13 is mounted in a buffer tube 15 and a jelly 11 is selectively filled therein. The plurality of optical fiber units 10 are inserted into a secondary sheath 16, and the secondary sheath 16 is filled with waterproof powder or jelly 17 for making it watertight. In addition, the outside of the secondary sheath 16 is selectively wrapped by a waterproof tape or paper 18, and the outermost layer is protected by a primary sheath 19 made of resin. Among them, the buffer tube plays a very important role of directly protecting the optical fiber from external forces, and it is also a most essential factor to determine properties of the optical cable.
The buffer tube is generally made of a crystalline polymer such as high density polyethylene and polypropylene, which is hard and has excellent mechanical characteristics, in consideration of a primary function for protecting an optical fiber. However, since the crystalline polymer has very high crystallinity, a shrinkage of the tube is greatly changed according to a cooling condition after the tube is extruded. If the tube is shrunk, bending of the optical fiber is caused, resulting in attenuation that deteriorates transmission capability and rate of the optical fiber. Thus, it is difficult to establish and control extrusion conditions of tubes to which a polymer resin with high crystallinity is applied. In addition, in case a gap between the optical fiber and the tube is small due to the small outer diameter of the unit tube and the high packing density of the optical fiber inserted into the tube, the aforementioned hard polymer materials are broken due to bad flexibility when a cable or tube is seriously bent. Accordingly, stress is given to the optical fiber to cause attenuation, and the tube does not easily restore its original shape due to low elasticity of the materials. Since the tube is not restored into an original shape, stress caused by pressing of the tube is continuously exerted to the optical fibers inserted into the tube. As mentioned above, the attenuation is increased due to the bending of the optical fiber caused by lateral pressure. In addition, these hard polymer materials are not easily shed and require experienced skill for shedding, so they take much time for contact of optical fibers. That is to say, some strands of optical fibers should be taken out to branch off the cable in a midway position. However, the buffer tube made of hard polymer materials such as high density polyethylene may damage the optical fiber when being shed, and it is also difficult to cut out the tube smoothly. In order to shed the buffer tube, an effective way is to make a scar on a certain region of the buffer tube and then tear it. However, the hard high density polyethylene has not good tear characteristic due to bad initial brittleness and strong tenacity. In addition, since the buffer tube is elongated while being torn, it is not easy to tear it over several ten centimeters smoothly.
In order to solve this problem, PE-EVA (polyethylene-ethylene vinylacetate copolymer) blend, plasticized PVC (polyvinylchloride), PVC-PUR (polyvinylchloride-polyurethane) blend, and polyolefin elastomer are recently used as a material of the buffer tube.
However, the plasticized PVC is apt to be easily deformed due to external force or lateral pressure, and it has low elasticity, so the buffer tube made of the plasticized PVC does not restore its original shape if it is pressed during installation. Thus, the plasticized PVC transfers stress to optical fibers as it is, thereby causing attenuation. In addition, the buffer tube made of PE-EVA blend or PVC-PUR blend has bad elasticity and weak jelly resistance, though it has better reliability than that made of plasticized PVC. Meanwhile, polyolefin elastomer has good elasticity but is too sensitive to external stress during the manufacturing procedure, so the buffer tube made of it may be easily elongated and generates after-shrinkage, thereby causing bending of optical fibers.